Fuel cells are devices in which the free energy of reaction released by the combination of a fuel (e.g. hydrogen, hydrogen-containing mixtures, alcohols, hydrocarbons) with an oxidant (e.g. pure oxygen or air) is not completely degraded to thermal energy, being converted instead to electric energy in the form of direct current. In such devices, the fuel is supplied at the anode, which assumes negative polarity, and the oxidant is supplied at the cathode, which conversely assumes positive polarity.
The generation of electric energy in fuel cells is of extreme interest in view of the high efficiency of utilisation of the fuels employed—as the process is not subject to the limitations of Carnot's cycle—and for the scarce environmental impact, in terms of noxious emissions and noise. When pure hydrogen is chosen as the fuel, such environmental impact virtually nought.
Fuel cells can be schematically classified in different types, essentially characterised by the type of electrolyte which separates the anodic and the cathodic compartment, and consequently by the temperature range in which they can be typically operated; this type of classification directly reflects on the effective or prospective utilisation of these systems. In particular, fuel cells that operate at high temperature, that is above 200° C., are already established as an alternative source of electric energy in big size power plants, also due to the attractive possibilities of co-generation allowed by the high thermal level. On the other hand, the kind of cells that appears more interesting in the field of small and medium size electric generation, both for stationary and for mobile (e.g. automotive) applications utilises a protonic conduction membrane as the electrolyte. It is known in the art that fuel cells utilising such membranes cannot be operated at temperature close to or higher than 100° C. as a strong decline in the protonic conductivity due to the difficulty of maintaining a high level of hydration occurs. For such reason, fuel cells utilising a polymeric membrane as the electrolyte are traditionally operated at a maximum temperature of 70–80° C. The high energy and power density associated to the utilisation of solid polymeric electrolytes and the rapidity in starting up and bringing to regime conditions the fuel cells employing the same, make the membrane fuel cells much more competitive of any contender for such type of applications. The limitation in the presently allowed thermal level nevertheless constitutes an important limitation to the full market affirmation of fuel cells employing commercial ion-exchange membranes: the possibility of operating at temperatures higher than 100° C. would in fact result in a mitigation of the requirements of the heat withdrawal circuits (a very important prerogative especially in applications of the automotive type), and in the possibility of improving the overall energetic yield by co-generation. Furthermore, the limitations given by the lack of flexibility in the fuel than can be supplied at temperatures below 100° C. must be taken into account. The availability of pure hydrogen is in fact limited to a selected niche of applications in which such fuel is present as a by-product, as in the case of sodium chloride electrolysis plant. In the case, for instance, in which hydrogen comes from the conversion of natural gas, alcohol or fossil fuels, the problem represented by the inevitable presence of carbon monoxide, simple traces of which are sufficient to sensibly penalise the functioning of the presently known anodic catalysts at the commonly employed temperatures, is well known. The fuel purification systems of practical application do not allow a decrease of the content of CO in the fuel below 10 parts per million (ppm); besides being very burdensome process-wise, going beyond such limit would have no practical meaning: actually, the carbon dioxide which constitutes, together with hydrogen, the product of the reactions of conversion of primary fuels (partial oxidation or steam reforming), coming in contact with the anodic catalyst of the fuel cell would in its turn form carbon monoxide in concentrations of that order of magnitude, being in chemical equilibrium therewith. 10 ppm of CO are largely sufficient to poison the commercially available platinum-based anodic catalysts to a significant extent. As the reaction of formation of the Pt-CO abduct is exothermal, the poisoning of the current platinum-based catalysts upon exposition to very small amounts of carbon monoxide may be virtually eliminated by increasing the operating temperature above 130° C. More sophisticated catalysts, for instance those based on platinum-ruthenium or platinum-molybdenum alloys, show an almost complete tolerance to such poisoning phenomena even at lower temperatures, for instance at 110° C. The commercial fuel cell membranes really developed to such an extent as to allow their effective use in industrial applications are made of perfluorocarbonsulphonic acids (for example Nafion®), for which the possibility of operating beyond 100° C. is seriously hindered, as said above, by the drastic decrease of protonic conductivity in the operating conditions. On the basis of protonic conductivity measurements carried out on such membranes in the range of 100–160° C. and at different conditions of relative humidity it has been determined that the decrease of protonic conductivity in fuel cells must essentially be related to the difficulty of maintaining the water balance of the system, especially when operating with gaseous reactants at low pressure (which is the inevitable case for the majority of practical applications to maximise the system efficiency). The above holds true also for other types of membranes proposed as an alternative to the perfluorocarbonsulphonic ones such as, for instance, those based on the sulphonation of polybenzimidazol (PBI), polyethersulphone (PES), or polymers of the family of polyetherketones (PEK and similar). Such membranes, though having a glass transition temperature higher than Nafion, and consequently a higher tolerance to high temperatures, require a very high water supply for their functioning, burdening the fuel cell operating conditions significantly; for instance, the sulphonated polyetheretherketone's (PEEK) based membranes, which are the closest to commercialisation among those cited due to their excellent chemical and mechanical properties, develop a sufficient ionic conductivity only when the relative humidity is higher than 95%. An improvement of the humidification conditions of perfluorocarbonsulphonic membranes has been proposed in the U.S. Pat. No. 5,523,181: in the case of fuel cells operating on hydrogen and oxygen or hydrogen and air, the dispersion of very fine particles of silica gel within the relevant polymers helps maintaining the water balance, allowing the operation of fuel cells either with a reduced external supply of humidity in the gaseous reactant flow or, in the most favourable cases, even without any water supply, that is relying on the sole product water. Such an improvement, although not allowing per se the operation of fuel cells above 100° C., mitigates to some extent the aforementioned dehydration phenomena. A further improvement of this concept is disclosed in the European patent application EP 0 926 754, wherein an appropriate thermal treatment effected on a membrane modified according to the teaching of U.S. Pat. No. 5,523,181 permits the operation at a temperature close to 150° C. Nevertheless, the modification of perfluorocarbonsulphonic membranes with silica presents some drawbacks: the insertion of a non conductive component through the whole thickness of the polymeric membrane may negatively affect the electrical efficiency of the whole system. Moreover, the method of production of the membranes is substantially altered and complicated: whereas, in the current industrial production, the membrane is extruded from the perfluorocarbonsulphonic acid having the functional groups in fluorosulphonic form and eventually hydrolysed, which allows a continuous cycle operation, when silica has to be embedded it is necessary to start from the ionomer in aqueous solution or suspension, for instance dissolving the prehydrolysed perfluorocarbonsulphonic acid at high pressure and temperature, mixing it with silica gel, and then depositing the membrane from the liquid phase. EP 0 926 754 discloses, for instance, the preparation of membrane samples by means of a Petri disk, that is with a method which cannot be scaled up to the industrial level. Besides this, to be able to operate at high temperature in stable conditions it is necessary to carry out an auxiliary thermal treatment beyond the glass transition temperature, controlling the process with an online diffractometer, that is through a further phase of difficult industrial operability.
It has thus been identified, with respect to the current state of the art, the need of a new invention allowing to operate the current polymeric membrane fuel cells at a temperature close to or higher than 100° C. (for instance in the range of 90–160° C.) without having to modify the currently commercially available membranes, preferably with a water supply requirement in the system substantially below the saturation level of the gaseous reactants.
It has further been identified the need of a means for operating a fuel cell in the presence of significant impurities present in the fuel (for instance, carbon monoxide in the order of tens of ppm) without giving rise to phenomena of poisoning or deactivation of the catalysts commercially available, even at temperatures close to 100° C.
It is an object of the present invention to provide a membrane-electrode assembly for polymeric membrane fuel cells capable of overcoming at least some drawbacks of the prior art.
According to one aspect, it is an object of the present invention to provide a membrane-electrode assembly for polymeric membrane fuel cells characterised by high protonic conductivity and high efficiency in conditions of reduced relative humidity of the reactants with respect to saturation, optionally in the presence of small amounts of carbon monoxide or other contaminants present in the fuel, and a method of production thereof.
According to another aspect, it is an object of the present invention to provide a gas diffusion electrode which allows operation of a state of the art membrane in conditions of reduced relative humidity of the reactants with respect to saturation, optionally in the presence of small amounts of carbon monoxide or other contaminants present in the fuel.
According to another aspect, it is an object of the present invention to provide a method of operation of a polymeric membrane fuel cell in conditions of reduced relative humidity of the reactants with respect to saturation, optionally in the presence of small amounts of carbon monoxide or other contaminants present in the fuel.
These and other objectives will be further clarified from the following description and examples.